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see
Instruction 6 for procedure.

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POWER OF ATTORNEY

KNOW ALL BY THESE
PRESENTS, that the undersigned hereby constitutes and appoints A. W. Homan, Karin L. Walker and Kevin J. Hickey, and their respective successors, and each of them the undersigned’s true and lawful attorneys-in-fact and agents with full power
of substitution, signing singly, for the undersigned and in the undersigned’s name, place and stead, in any and all capacities, to:

1.

prepare, execute in the undersigned’s name and on the undersigned’s behalf, and submit to the U.S. Securities and Exchange
Commission (the “SEC”) a Form ID, including amendments thereto, and any other documents necessary or appropriate to obtain codes and passwords enabling the undersigned to make electronic filings with the SEC of reports required by Section
16(a) of the Securities Exchange Act of 1934 (as amended, the “Exchange Act”) or any rule or regulation of the SEC;

2.

complete, execute and timely file for and on behalf of the undersigned, in the undersigned’s capacity as an officer and/or
director of Prothena Corporation plc (the “Company”), Forms 3, 4 and 5, any other forms, and any amendments thereto in accordance with Section 16(a) of the Exchange Act and the rules and regulations thereunder; and

3.

take any other action of any type whatsoever in connection with the foregoing which, in the opinion of such
attorney-in-fact, may be of benefit to, in the best interest of, or legally required by, the undersigned, it being understood that the documents executed by such attorney-in-fact on behalf of the undersigned pursuant to this Power of
Attorney shall be in such form and shall contain such terms and conditions as such attorney-in-fact may approve in such attorney-in-fact’s discretion.

The undersigned hereby grants to each such attorney-in-fact full power and authority to do and perform any and
every act and thing whatsoever requisite, necessary or proper to be done in the exercise of any of the rights and powers herein granted, as fully to all intents and purposes as the undersigned might or could do if personally present, with full power
of substitution or revocation, hereby ratifying and confirming all that such attorney-in-fact, or such attorney-in-fact’s substitute or substitutes, shall lawfully do or cause to be done by virtue of this power of attorney and the rights and powers
herein granted. The undersigned acknowledges that the foregoing attorneys-in-fact, in serving in such capacity at the request of the undersigned, are not assuming, nor is the Company assuming, any of the undersigned’s responsibilities to comply with
Section 16 of the Exchange Act.

The undersigned agrees that each such attorney-in-fact herein may rely entirely on information furnished orally or
in writing by the undersigned to such attorney-in-fact. The undersigned also agrees to indemnify and hold harmless the Company and each such attorney-in- fact against any losses, claims, damages or liabilities (or actions in these respects) that
arise out of or are based upon any untrue statements or omission of necessary facts in the information provided by the undersigned to such attorney-in-fact for purposes of executing, acknowledging, delivering or filing Forms 3, 4 or 5, Form ID or any
other form (including amendments thereto) and agrees to reimburse the Company and such attorney-in-fact for any legal or other expenses reasonably incurred in connection with investigating or defending against any such loss, claim, damage, liability
or action.

This Power of Attorney supersedes any power of attorney previously executed by the undersigned regarding the
purposes outlined in the first paragraph hereof (“Prior Powers of Attorney”), and the authority of the attorneys-in-fact named in any Prior Powers of Attorney is hereby revoked.

This Power of Attorney shall remain in full force and effect until the undersigned is no longer required to file
Forms 3, 4 and 5 with respect to the undersigned’s holdings of and transactions in securities issued by the Company, unless earlier (a) revoked by the undersigned in a signed writing delivered to the foregoing attorneys-in-fact or (b) superseded by a
new power of attorney regarding the purposes outlined in the first paragraph hereof dated as of a later date.

IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be executed as of this 5th day
of June, 2019.